The present invention relates generally to automatic white adjusting circuits for television cameras, and more particularly to an automatic white adjusting circuit for a television camera in which the adjustment of the white is performed automatically by the simple operation of directing the television camera towards a white surface.
Conventionally, in a high-grade television camera for business use, an automatic white adjusting circuit is provided to automatically adjust the white (correct the color temperature), so that the tint of an output signal of the color television camera does not change due to the color temperature of the illumination light source. This automatic white adjusting circuit uses a theory that, upon picking up an image of a white object, the three primary colors of red, green, and blue lights must be of the same level, and adjusts the respective output signals of the three primary colors so that the three levels become the same.
There have been many kinds of conventional automatic white adjusting circuit in which an analog or a digital system is used. However, they all suffered disadvantages in that they all possessed a plurality of operating buttons which were troublesome to operate, and in the analog systems, there was a disadvantage in that the automatic white adjusting circuit as a whole became costly because of the high cost of the analog memories. Therefore, it was not possible to apply the automatic white adjusting circuit of this television camera for business use, to the desired television camera for home use.
Furthermore, the television camera for business use is designed under the assumption that it will be operated by a professional operator, and requires the adjustment of the white every time the voltage source is applied to the television camera. Thus, also from this point of view, the above automatic white adjusting circuit was not applicable to the television camera for home use.
A conventional portable color television camera is supplied with voltage from a voltage source of a portable video tape recorder. This type of a portable video tape recorder shuts the voltage source off, when the tape moving mode is set to the stop mode, to reduce the power consumption. Accordingly, when the video tape recorder is set to the stop mode, the voltage source of the television camera is also cut off.
Therefore, in the automatic white adjusting circuit using digital memories, there is a disadvantage in that the content of the digital memory is destroyed when the voltage source is shut off, requiring the adjustment of the white every time the image pick-up operation is performed. Accordingly, the operation becomes troublesome, not being able to perform the image pick-up operation on a preferable timing.
To overcome the above described disadvantages, the television camera could possibly be provided with its own battery, so that the content of the digital memory can be kept in the same state, even when the video tape recorder is in the stop mode. However, by providing the television camera with its own battery, both the weight and the size of the television camera increase, thus resulting in the disadvantage in that, reducing of the weight and size demanded from a portable television camera cannot be met.
On the other hand, when the automatic white adjusting circuit is constructed by using analog memories, the television camera does not require its own battery since the analog memory content does not change when the voltage source is put on or off. But the content of the analog memory is slowly destroyed with time due to natural discharge, and thus requires a circuit to reduce this natural discharge. This results in high cost, suffering a disadvantage in that, it is not suitable for television cameras for home use from which low cost is demanded.
Therefore, an automatic white adjusting circuit which is especially effective when applied to a color television camera for home use, comprising simple circuit construction and in which the white is automatically adjusted by simple operation, was highly desired.
In the color television systems presently being used such as the NTSC, PAL, or SECAM systems, they are standardized to transmit a luminance signal and two color difference signals (I- and Q-signals in the NTSC system, and (R-Y) and (B-Y) signals in the PAL system). The white is obtained when the two color difference signals are zero, and the color television camera for home use comprises a circuit for generating the above color difference signals.
Upon picking up of an image of an entirely white object by a television camera, the color difference signals are zero when the white adjustment matahes with the color of the illumination light source, and the output of the above color difference signal generating circuit deviates towards the positive or negative side when the white adjustment does not match with the color of the illumination source. To adjust this deviation towards the positive or negative side to zero, the primary color signal from the light element of any of the two primary colors of the three primary colors red (R), green (G), or blue (B) can be increased or decreased.
In the circuit for generating the color difference signals (R-Y) and (B-Y), for example, the amplification rate of the primary color signals of R and B can be gradually decreased when the generating output signal increases towards the positive side. On the other hand, the amplification rate of the primary color signals of R and B (referred as R-signal and B-signal hereinafter) can be gradually increased, when the generating output signal increases towards the negative side. The (R-Y) and R-signals, and the (B-Y) and B-signals, respectively, do not completely coincide with each other, however, since the R-signal contributes most to the (R-Y) signal and the B-signal contributes most to the (B-Y) signal, the levels of the primary color signals of R and B can be adjusted accordingly, to respectively adjust the two color difference signals (R-Y) and (B-Y) to zero.
However, upon actual operation, there is no problem when an entirely white surface exists under the illumination light source, but outdoors, in most cases, a white plate which is an object of an entirely white surface is not on hand. When a scenery having each color in a balanced manner is picked up by a television camera in this case, positive or negative signals appear in the signal obtained from the color difference signal generating circuit of the television camera, where the positive or negative signals have the color difference signal obtained upon picking up of a white plate under that particular illumination light source as their centers. In most cases, the average value of the color difference signal at that time can be taken as the color difference signal obtained upon picking up the image of a white plate under that illumination light source.
There are also times when the white surface is too small for the enlargement to the full scale of the image pick up field of vision, and in this case, it is convenient to use the color difference signal relative to one portion of the picture, such as the central portion of the picture.
Accordingly, in a U.S. patent application Ser. No. 183,392 filed Sept. 2, 1980, entitled "AUTOMATIC WHITE ADJUSTING CIRCUIT FOR A TELEVISION CAMERA" in which the assignee is the same as that of the present application, a circuit was proposed which is capable of simply performing white adjustment. In this previously proposed circuit, average values are obtained by the use of a low-pass filter for the partial or entire duration of the picture of the two color difference signals, to adjust the television camera so that it considers the color of the light source as white, and compares the average values with a reference level (zero color difference signal level) which is equal to the two color difference signal levels from the color difference signal generating circuit upon picking up an image of an entirely white surface, to adjust the average values of the color difference signals to the above reference level by independently increasing or decreasing each of the two color difference signal level. For the white adjustment, the output signal of the circuit which compares the above average values of the color difference signals with the reference level, is used to generate the control voltage of a variable gain amplifier provided in each of the two primary color signal transfer systems, and the control voltages are fixed when the average values of the color difference signals become of the same level as the above reference level.
Furthermore, the above proposed automatic white adjusting circuit, two reversible counters are used as control voltage generating devices, and converts the total output (parallel bits) of these reversible counters from digital to analog form, to obtain the above control voltages. The control voltages are increased or decreased according to the addition count-up or subtraction count-down performed by the reversible counters, as a result of the comparison performed between the above color difference signals and the reference level. The above reversible counters alternately repeats a one-count addition and a one-count subtraction, when the average values of the two color difference signals approach the reference level, to generate control voltages which oscillate above and below the reference level, and fixes (maintains) the level of the control voltages by stopping the counting operation of the reversible counters. According to this proposed automatic white adjusting circuit, the cost as well as the size of the circuit are reduced.
However, in the above described previously proposed circuit, the difference between the most suitable control voltage (reference level) and the values of the control voltages which oscillate above and below the most suitable control voltage must be small so that problems are not introduced from the practical point of view. Accordingly, when the number of bits in the above reversible counters are small, the control voltages respectively reach the two values above and below the most suitable control voltage which are closest to the most suitable control voltage within a short period of time, however, white adjustment of high quality cannot be performed in this case. On the other hand, when the number of bits in the above reversible counters are large, white adjustment of high quality can be performed, however, the time required for the control voltages to respectively reach the two values above and below the most suitable control voltage which are closest to the most suitable control voltage become long.
Therefore, in a case where the above reversible counters are respectively constructed from two four-bit counters, the difference between the most suitable control voltage and the values of the control voltages which oscillate above and below the most suitable control voltage becomes practically negliglible. However, when a pulse of 60 Hz (50 Hz in the case of the PAL, or SECAM system) which is synchronized with the vertical deflection pulse, is used as the clock pulse, the maximum time for the control voltages above and below the most suitable control voltage to reach the closest values with respect to the most suitable control voltage becomes 1/60.times.(2.sup.8 -1)=4.25 (seconds). In addition, the operator of the television camera must continue to operate a switch and the like for operating the counting operation of the reversible counters, until the counting operation of the reversible counters are stopped and the control voltages are fixed (maintained constant). Hence, in this proposed system, there was a problem in that the television camera became quite troublesome to operate.